Yes - he is the only one
Yea my Q tank is used before I put them in but again he has been in main tank for 2 mths with nothing added since him. I keep all fish in Q tank for a week before adding them, some do it longer but I figure after a week with no issues then it's probably my faught at that point
If you only lost the powder blue I doubt it was velvet. If it was ich I would think your hippo would show signs by now.
I will never add another fish without first treating them and closely monitoring them for at least 21 days.